As is known, during the operations of landing of aircraft, the latter have to be braked in order to reduce their speed and terminate the trip safely within the landing strip. Braking of the aircraft occurs by the action of brakes of an aerodynamic type and by the action of mechanical brakes coupled to the wheels of the undercarriage of the aircraft.
Currently, undercarriages of aircraft are provided with mechanical disk brakes, which are operated by oil under pressure coming from a hydraulic circuit. In particular, the braking action is modulated manually by the pilot by action on a brake pedal that acts on valves of the hydraulic circuit.
As is known, disk brakes are subject to a rapid wear on account of the high value of kinetic energy that is to be dissipated during braking of the aircraft.
Disk brakes moreover reach very high temperatures, which can jeopardize the efficiency thereof and drastically reduce their service life.
There have moreover been proposed braking systems of an electrical type, which use reversible electrical machines, directly coupled to the undercarriage, which are designed to provide a braking action of a “totally electrical” type.
For example, the patent application No. PCT WO 2005/102839 describes an axial-flux machine directly coupled to the wheels of an undercarriage of an aircraft in order to provide a plurality of functions, amongst which:                prior to the landing phase the electric motor is supplied so as to set the wheels of the undercarriage in rotation and favour landing, moreover reducing the wear of the tyres due to the effect of friction on the landing strip;        following upon contact of the wheels of the undercarriage with the landing strip, the reversible electrical machine behaves as a generator, producing energy and thus exerting a braking action—the electrical energy is dissipated in resistors or is supplied to sections of the motor that provide a braking action opposite to the direction of rotation of the generator;        part of the energy is stored in an on-board system for being re-used subsequently; and        following upon completion of the operations of landing of the aircraft, the reversible electrical machine can be supplied and used on the runway for moving the aircraft in opposite directions.        
The applicant of the patent application has found how, notwithstanding the fact that the solution disclosed in the patent application No. PCT WO 2005/102839 referred to above can be acknowledged absolute theoretical validity, it cannot be implemented on any commercial aircraft in operation. In fact, from an analysis of the dimensions of the rims of the wheels for aircraft undercarriage and of the values of the torques necessary to obtain safe braking (i.e., in the times and in the ways required by current certification standards), it emerges that at present the rims of the undercarriage wheels do not enable in any way integral housing inside them of machines capable of generating adequate braking torques in so far as the radial dimensions are limited by the internal diameter of the rim of the wheels of the undercarriage within which the reversible electrical machine is to be integrally housed.